1. Field of the Invention
The present invention relates to a thermal head mounted on a thermal-transfer printer or the like and a manufacturing method of the thermal head.
2. Description of the Related Art
FIG. 6A is a cross-sectional view showing a thermal head 100 having a so-called folded electrode structure, and FIG. 6B is a plan view of the thermal head 100 (an abrasion-resistance protective layer is excluded). The thermal head 100 includes a heat dissipation substrate 102, and on the heat dissipation substrate 102, the thermal head 100 also includes a heat storage layer 103, a plurality of heating resistors 105 (105a and 105b) which generate heat by electricity supply, separate electrodes 107 connected to the respective heating resistors 105, a common electrode 108 commonly connected to the heating resistors 105, U-shaped folded electrodes 111 each connected to one end of a pair of heating resistors 105a and 105b which are disposed adjacent to each other, and an abrasion-resistance protective layer 110. In this thermal head 100, the pair of heating resistors 105a and 105b connected to each other with the folded electrode 111 forms one printing dot portion.
The thermal head 100 is formed, for example, by the following process.
First, a resistor layer 104 and an Al electrode layer E are formed over the heat storage layer 103 provided on the surface of the heat dissipation substrate 102. Next, part of the Al electrode layer E and part of the resistor layer 104 are removed so as to form patterns of the folded electrodes, the separate electrodes, and the common electrode, which are to be formed. The Al electrode layer E is formed to have a thickness of approximately 1 μm in order to decrease the electrode resistance (in order to suppress the increase in electrode resistance caused by decrease in head size). By this patterning, a width dimension W′ of the heating resistor is determined. Subsequently, part of the Al electrode layer E is removed so as to form opening portions a through which the surface of the resistor layer 104 is exposed. Areas of the surface of the resistor layer 104, which are exposed through the openings, each form the heating resistor 105, and a length dimension L′ of the heating resistor is determined by the opening portion α. The Al electrode layer E is separated by each opening portion a into the U-shaped folded electrode 111, which is connected to one end side of the pair of the adjacent heating resistors 105 (105a and 105b), and the separate electrode 107 and the common electrode 108, which are connected to the other end side of the pair of the adjacent heating electrodes 105a and 105b and which extend in the same direction. Next, the abrasion-resistance protective layer 110 is formed so as to cover the heating resistors 105, the folded electrodes 111, the separate electrodes 107, and the common electrode 108. Since the thickness of the Al electrode layer E is large, such as approximately 1 μm, steps are formed at the two ends of the opening portion α, that is, at the boundaries of the heating resistor 105 with the folded electrode 111, the separate electrode 107, and the common electrode 108, and these steps form a step portion 110a in the surface of the abrasion-resistance protective layer 110. When the steps are present in the vicinity of the heating resistor 105, since contact efficiency between a print medium and the heating resistance 105 is degraded, the step portion 110a of the abrasion-resistance protective layer 110 is polished so as to smooth the contact surface with the print medium. Accordingly, the thermal head 100 can be obtained.
The resistance of the above heating resistor 105 is largely dependent on the planar shape (aspect ratio L/W) thereof. However, in order to determine the planar shape of the heating resistor 105 in a manufacturing process which has been performed, since patterning must be performed twice respectively for the length dimension L′ and for the width dimension W′, the deviation is generated between pattering steps, and thereby the resistance of the heating resistor 105 varies. Accordingly, a high-performance thermal head has been desired which has small variation in resistance between heating resistors by accurately determining the planar shapes thereof.